In the blue half-light of the Antarctic autumn, a thin film spreads across the continent’s coastal waters. It’s an embryonic form of sea ice: a mush of microscopic crystals that floats on the dense, salty water of the Southern Ocean.

As winter takes root, this proto-sea ice grows thicker and stronger until it encircles Antarctica in a vast frozen ring. The ice spans nearly 7 million square miles at its peak, an area roughly twice the size of the United States.